1. Field of the Invention
The present invention relates to an image forming apparatus having a plurality of arrayed illuminants as an exposing device wherein the plurality of arrayed illuminants are controlled dynamically for lighting so as to form an image on a latent image bearing body (e.g. a photosensitive body drum) by exposing an image.
2. Description of the Related Art
An image forming apparatus which forms an electrostatic latent image corresponding to an exposed image focused on the generating line of a photosensitive body (an image bearing body) by controlling dynamically for lighting a plurality of arrayed illuminants aligned along a main scanning direction according to an image data at a n-bit unit (n is an integer) or a main scanning line unit is publicly known.
For example, arrayed illuminants are arrayed LEDs having a plurality of LED elements. An image forming apparatus using such arrayed LEDs is called an LED printer wherein an exposing head comprises a plurality of arrayed LEDs. As shown in FIG. 22, an LED print head 12 is disposed along a main scanning direction and a photosensitive body drum 11 is moved in an auxiliary scanning direction at a linear speed (a printing speed) of S mm/s while a dynamic lighting control where each of arrayed LEDs is lighted sequentially is performed so as to form an electrostatic latent image on the photosensitive body drum 11.
Meanwhile, according to the dynamic lighting control stated above, when time-shared exposures are done to the photosensitive body drum 11, which is rotating at a printing speed S, an image is exposed obliquely at an arrayed LEDs unit to the aligning direction of arrayed LEDs.
M (m is an integer greater than 1) sets of arrayed LEDs (arrayed LEDs 35-1˜35-m) are provided and these arrayed LEDs 35-1˜35-m are disposed in a main scanning direction as shown in FIG. 23. As the photosensitive body 11 is rotated in the bold arrow direction, if sets of arrayed LEDs is controlled to be lighted in the sequence of 35-1˜35m, the drum 11 is rotated at the time when the 35-m is lighted more than at the time when the 35-1 is lighted. As a result, an actual exposed line is drawn obliquely below an ideal exposed line shown in a dashed line in the figure. The problem is the exposed line becomes oblique so that it is difficult to obtain a desired image with the image forming apparatus in which sets of arrayed LEDs are controlled to be lighted in a time-shared manner by the dynamic lighting control. In order to overcome this drawback, Japanese patent publication JP1994-14610 discloses a circuit for controlling a pulse width, which corrects a deviance of the center position of printed pixels caused by rotation of the photosensitive body drum in an LED printer using a plurality of arrayed LEDs as a light source to improve printed image quality.
However, although, according to the prior art, a deviance of the center position of printed pixels caused by rotation of the photosensitive body drum is corrected, unless an exposing head is accurately fixed to the LED printer (a printer housing), a deviance is also generated between an ideal exposing line and an actual exposing line depending on an accuracy of fixing the print head. In other words, such drawback is not overcome as an image is obliquely exposed for arrayed LEDs unit. Therefore, an exposing head needs to be fixed with a precise accuracy at a predetermined position when it is fixed to an LED printer (a printer housing).
Further, in a tandem type LED printer, a plurality of exposing heads needs to be accurately fixed so as to be accurately parallel with each of the exposing heads or else exposing lines are out of alignment for each of the exposing heads even if the oblique exposure is corrected, which results in being incapable of forming a good image. Therefore, when an exposing head is fixed, the fixing accuracy is severely required. However, workload for mounting is up in order to fix the print heads accurately, increasing production cost.
Thus, a conventional LED printer has a problem that an oblique exposing is performed with respect to the aligning direction of arrayed LEDs, an actual exposing line deviating from an ideal exposing line due to the fixing accuracy of the LED print head. Parallelism of exposing lines between LED print heads is deteriorated unless each LED printhead is accurately fixed, so that it is difficult to obtain required images.
When light intensity is low for a printing speed or sensitivity of a photosensitive body drum is low in the aforementioned LED printer in which m (m is an integer greater than 1) sets of arrayed LEDs having n (n is a positive integer) elements of LED are disposed in a main scanning direction and the sets of arrayed LEDs are controlled to drive by dynamic lighting control, appropriate exposing energy (light intensity) can not be obtained.
Accordingly, m sets of arrayed LEDs are divided into a plurality of groups and each group is controlled by dynamic lighting control. For example, as shown in FIG. 19, m sets of arrayed LEDs are divided into tow groups (a first and a second group) each of which is provided with a dynamic lighting circuit and controlled by dynamic lighting control.
Referring to FIG. 19, an exposing head has m sets of arrayed LEDs from a first set to an m'th set. P sets of arrayed LEDs from a first set to a p'th set, i.e. 35-1˜35-p, belong to a first group and m-p sets of arrayed LEDs from a (p+1)'th set to a m'th set, i.e. 35-(p+1)˜35-m, belongs to a second group, where p is an integer greater than 1 and less than m, the first group contains the same sets of arrayed LEDs as the second group and each set of arrayed LEDs from the first to the m'th has n elements 12-1˜12-n of LEDs.
The p sets of arrayed LEDs from the first set to the p'th set, i.e. 35-1˜35-p, are driven by a first LED driver circuit and the m-p sets of arrayed LEDs from the (p+1)'th set to the m'th set, i.e. 35-(p+1)˜35-m, are driven by a second LED driver circuit. Thus, the p sets of arrayed LEDs from the first set to the p'th set, i.e. 35-1˜35-p, are connected to a first anode driver 13 and a first cathode driver 14 and the m-p sets of arrayed LEDs from the (p+1)'th set to the m'th set, i.e. 35-(p+1)˜35-m, are connected to a second anode driver 15 and a second cathode driver 16. The p sets of arrayed LEDs from the first set to the p'th set, i.e. 35-1˜35-p, are driven by the first anode driver 13 and the first cathode driver 14 based on an image data and the m-p sets of arrayed LEDs from the (p+1)'th set to the m'th set, i.e. 35-(p+1)˜35-m, are driven by the second anode driver 15 and the second cathode driver 16 based on an image data.
The first anode driver 13 has n output terminals from a first to an n'th terminal and each of the output terminals from the first to the n'th is connected to each of anodes of the LED elements of 12-1˜12-n. While, the first cathode driver 14 has p output terminals from a first to p'th terminal and each of the output terminals from the first to the p'th is connected to each of the arrayed LEDs of 35-1˜35-n. Viz., each of the first to the p'th output terminals of the first cathode driver is connected to each of the cathode of the LED elements in a set of the arrayed LEDs of 35-1˜35-p. Likewise, the second anode driver 15 has the output terminals from the first to n'th. In the arrayed LEDs 35-(p+1)˜35-m, each of the first to the n'th output terminals is connected to each of the anodes of the LED elements of 35-1˜35-n. The second cathode driver 16 has the first to the p'th output terminals each of which is connected to the (p+1)'th˜the m'th set of arrayed LEDs, 35-(p+1)˜35-m.
In the first group, sets of 35-1˜35-p arrayed LEDs are controlled to light dynamically as a sequence of the first to the p'th set. In the second group, sets of 35-(p+1)˜35-m arrayed LEDs are controlled to light dynamically as a sequence of the (p+1)'th to the m'th set. In this way, a latent image region on the photosensitive body drum is divided into two parts of the left and the right in the main scanning direction so as to prolong an exposing time at every set of arrayed LEDs in order to ensure an appropriate exposing energy.
In the meantime, when dynamic lighting control is done at every group by dividing arrayed LEDs into a plurality of groups, a stepped deviance of exposure depending on resolution is generated between areas divided by a plane A shown in FIG. 19 (a boundary plane between the arrayed LEDs 35-p and the arrayed LEDs 35-(p+1)). As shown in FIG. 20, in the first group, when a dynamic lighting control is sequentially done from the first set of arrayed LEDs 35-1, light is exposed obliquely in an aligning direction of sets of arrayed LEDs at an arrayed LED unit (lowering to the right as shown in FIG. 20) because of time sharing exposure to the photosensitive body drum rotating at a printing speed of S. While, in the second group, since a dynamic lighting control is sequentially done from the (p+1)'th set of arrayed LEDs, 35-(p+1), the stepped deviance of exposure is inevitably generated at the boundary plane A so that a good image can not be obtained.
An exposing time for one scanning T line needs to be determined within a sheet transporting time under a resolution so that an exposing time is determined as satisfying the relationship, T line(s/line)<1/{S(mm/s)×resolution(dot/line)}×25.4. Therefore, when the above relationship is satisfied and the exposing time T line is made maximum (under the condition of a maximum exposing time), a stepped deviance of exposure for a resolution is generated.
As a result of generating the above mentioned stepped deviance, when lines vertical to the sheet transporting direction are alternately repeated to print, a boundary part showing the stepped deviance is emphasized as if a vertical line appears to exist according to a characteristic of human visual perception as shown in FIG. 21.
Whatever the case may be heretofore, a good image cannot have been formed, when dynamic lighting control is done at every group by dividing arrayed LEDs into a plurality of groups.